Title:
Truthfully, I like the title. I never have heard of the word "Heartsigh" and thought you just forgot to put a space until I read the description. It's a pretty unique title compared to many I've seen and I think it's good for compelling people to read.

Description/Foreword:
It sounds cliche from just the description. The whole "golden boy" and being good at almost everything is something that would turn me away. It's been done before a bit too much. I've also read this line "both of them don't realise (it should be realize, as it is spelled with a z) that it will turn everything they know upside down." way too much in different contexts. So as for the description, it would turn me away unlike the title which would bring me in. Though, many people on here like cliches (I've come to notice since many fanfics that are popular are kind of cliche) so, other people would probably read this (unless they think this kind of ship is... odd(?) like I do) but for me, I wouldn't as far as the description goes.

Plot:
(This is before reading): So far, as I said above, the description makes me think it's going to be very cliche. The guy can do almost anything and excels then in comes the female OC/Idol (these are usually used in OC fanfics, I've noticed) who changes things. I mean, the whole vampire thing is a twist to this kind of cliche plot but the vampire thing is quite overdone as well. 

While reading: I like how it started, not going to lie. The whole "compared water to wine" comparison was very clever and gives in insight to what human blood vs animal blood is (because I've never thought about it like that). The way they meet was kind of eh, predictable. I kind of saw it coming especially at the end when they had a new classmate. So far, it's kind of cliche in story as well and I'm hoping that'll change but I can't tell as of right now. They keep meeting as well which I think is kind of predictable as well. They meet, they keep finding each other, then all the feelings and stuff happens. Then they can't stop thinking about each other and so on.

I didn't read all the chapters cause there's a bit much for me but as far as I see it (which is up to chapter six) it's kind of predictable on some things. Like when chapter six started with "Jungkook found himself thinking about Seulgi." I just kind of wasn't feeling it, it was too predictable. But overall, the story isn't that bad, it's fairly decent actually. 

Writing Style/Flow:
The writing style is pretty much perfect, the dialogue is very well executed and I like the detail added to certain things. As for grammar, I didn't see any mistakes (but I miss mistakes in my own work all the time) minus the word realize being spelled as realise. As for flow, it's on and off for me - it's kind of fast for my liking but any slower and the story might've become boring so I somehow think the flow is good but certain things (like Jungkook already thinking about Seulgi that early in meeting) felt too quick for my personal liking.

Characterization:
Seulgi seems like a decent character, not too cliche/mary sue like which is good. For her being a vampire, I think it's interesting the back story you gave with the vampires who help other vampires move when they're in need, that was definitely a unique concept and it gave her a little more character.

As for Jungkook, the description kind of tipped me off on his character. He kind of seemed like too much of a perfect character with the 'excels at sports, is well-liked, great at studying', I just wasn't into that but after reading the story, he isn't as bad as I figured. 

Personal Enjoyment:
Personally, I don't get why people ship this kind of stuff. I've always had a dislike for RV as a whole (don't worry though, I don't do biased reviews if that made you think so) and I don't see the point of shipping them with BTS/A group not from their own company but that's just me so for personal enjoyment, the pairing drove me away since I just don't like it at all.

As for the story, I think if it was a different pairing, I could see myself  maybe reading it, really doubt it though, (I don't read straight fanfics either... well, I barely have time for much fanfic reading since I busy myself but that's not the point). Though, the vampire concepts are usually quite overdone as well. After reading, I don't think this is a story I'd really look at past the description, unless it was a ship I read everything to which is rare, but it's a pretty well done fanfic overall. It may not be my cup of tea but it definitely is someone else's.